The present invention relates in general to a thin film device for use as a high specific energy electronic device, such as a capacitor, and a process for its manufacture. Specifically, the electronic device and method for manufacturing the electronic device involves hydrothermal deposition of a predominantly vertically oriented columnar (crystal) structured high dielectric constant film including an insulating filler material.
Useful inorganic materials with high dielectric constants are usually piezoelectric, but certain electrostrictive, ferroelectric, or anti-ferroelectric materials may be used for some applications. A common material with a high relative dielectric constant of much greater than 100, depending on composition, is lead zirconium titanate (hereinafter sometimes abbreviated as PZT). PZT is also strongly piezoelectric, and thus is also used in many electromechanical applications. Thin films of PZT are formed by various methods including physical vapor deposition (PVD) techniques such as sputtering, chemical vapor deposition (CVD) techniques, and chemical solution methods including sol-gel deposition. The chemical solutions may be applied for example by spin coating which is followed by a typical heat treatment (sintering) at a high temperature of 500–1000° C. to evaporate any solvent and to convert metal-organic precursors to inorganic materials. “Thick” film deposition methods, which are best used for films greater than about 10 microns thick, although thinner films of poorer quality have been used in commercial products, involve applying a mixture of powdered ceramic in an organic vehicle to a substrate and firing at very high temperature, at least 800° C., but preferably at least 1100° C. to obtain films with dielectric constants closer to bulk values. For reference, “bulk” material refers to the best available macroscopic sample with the same or similar material chemistry. Typically, because of the extremely high sintering temperatures used in the heat treatment, expensive electrode alloys of palladium or platinum are usually needed for best results.
The above-mentioned conventional piezoelectric thin film deposition methods are typically not economical for film thicknesses greater than one to two microns (also known as micrometers), and furthermore the thickest of such films can suffer from defects such as stress cracking. The “thick” film deposition methods produce relatively poor quality films, and furthermore require relatively expensive electrode materials.
Another approach for increasing the thickness of piezoelectric films is based on the use of hydrothermal synthesis which permits the intended reaction to proceed at a relatively low temperature (for example less than about 250° C.). Additionally, using the hydrothermal synthesis technique and low deposition temperatures a reduction in the electrode cost can be realized by using less expensive electrode materials. Previously reported hydrothermal synthesis techniques involve growing crystal of a piezoelectric material such as PZT on a compatible seed layer, for example titanium oxide, in a reactor with reagents containing for example Pb, Zr, and Ti, and a mineralizer such as potassium hydroxide, and heated to moderate temperatures of typically 120 degrees to 160 degrees C. Thick films can be formed at low temperatures by the hydrothermal synthesis technique, but the crystal grains produced are dependent on the orientation of the seed crystals, so that nearly randomly oriented seed crystals will produce a relatively low density film.
Accordingly, it is considered desirable to develop thin film devices (capacitors) with high specific energy, comparable to that of other capacitors such as aluminum electrolytic, or multi-layer ceramic capacitors, yet with lower energy loss than the aluminum electrolytic and lower manufacturing costs than the multi-layer ceramic capacitors. Current multilayer ceramic capacitors are manufactured using “thick” film methods such as screen printing or tape casting, thus such ceramic capacitors suffer from poor performance relative to bulk ceramics because the films are not fully dense, so that the resulting dielectric constant is typically less than one-half that of bulk.